| Summary: | Marine biogenic materials such as corals, shells, or seaweed have long been recognized as recorders of environmental conditions. Here, the bivalve Cerastoderma edule is used for the first time as a recorder of past seawater contamination with anthropogenic uranium, specifically 236 U. Several studies have employed the authorized radioactive releases, including 236 U, from nuclear reprocessing plants in La Hague, France, into the English Channel, and Sellafield, England, into the Irish Sea, to trace Atlantic waters and to understand recent climate induced circulation changes in the Arctic Ocean. Anthropogenic 236 U has emerged over recent years as a new transient tracer to track these changes, but its application has been challenged owing to paucity of fundamental data on the input (timing and amount) of 236 U from Sellafield. Here, we present 236 U/238U data from bivalve shells collected close to La Hague and Sellafield from two unique shell collections that allow the reconstruction of the historical 236 U contamination of seawater since the 1960s, mostly with bi-annual resolution. The novel archive is first validated by comparison with well-documented 236 U discharges from La Hague. Then, shells from the Irish Sea are used to reconstruct the regional 236 U contamination. Apart from defining new, observationally based 236 U input functions that will allow more precise tracer studies in the Arctic Ocean, we find an unexpected peak of 236 U releases to the Irish Sea in the 1970s. Using this peak, we provide evidence for a small, but significant recirculation of Irish Sea water into the English Channel. Tracing the 1970s peak should allow extending 236 U tracer studies into the South Atlantic Ocean.
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